Alkylated phenols are widely employed as antioxidants in a broad spectrum of products, spanning the gamut from foods to fuel oils. Among such alkylated phenols it has been found that 2,4,6-trialkylphenols are especially effective antioxidants in fuel oils, and in particular 2,4,6-triisopropylphenol is an excellent stabilizer for a broad range of fuel oils, including gasoline.
A characteristic displayed by alkylation processes used in the preparation of, for example, 2,4,6-triisopropylphenol from propylene and phenol is that 2,6-diisopropylphenol in particular, and 2,6-dialkylphenols in general, are by-products which are generally much more difficult to alkylate in comparison to other positional isomers. Since the effectiveness of the 2,4,6-trialkylphenols as antioxidants generally surpasses that of the 2,6-dialkylated phenol it is very desirable that any preparative method maximize the formation of 2,4,6-trialkylphenol. In this context it must be emphasized that even a relatively small improvement in the yield of the 2,4,6-trialkylphenol represents a substantial advance in the art and is of commercially great importance.
Silica-alumina combinations have been used as a catalyst in the alkylation of phenols with olefins. U.S. Pat. No. 2,684,389 teaches the use of a silica-alumina composite which is predominantly silica to alkylate cresol with isobutylene at a temperature less than about 180.degree. C. In U.S. Pat. No. 3,631,120 the patentee uses ammonium ion exchanged, calcined crystalline zeolite with a silica-alumina ratio between 4 and 4.9 as an alkylation catalyst. A composite of silica-alumina doped with an alkaline earth metal is disclosed as a catalyst in alkylation with isobutylene in U.S. Pat. No. 3,470,257, and a similar composite doped with a nitrogen-containing Lewis base is similarly utilized by the same patentee in U.S. Pat. No. 3,683,030. A process for preparing high purity p-tert-butylphenol using isobutylene oligomers as an alkylation agent with a silica-alumina catalyst in the presence of water is taught in U.S. Pat. No. 4,166,191.
This application discloses a method of trialkylating phenol with an olefin using a silica-alumina composite as a catalyst. More specifically, the invention is a method of trialkylating phenol with an olefin over a silica-alumina composite containing 40-60% silica at a temperature in the range from about 150.degree. to about 300.degree. C. In a still more specific embodiment the olefin is propylene.
An advantage of the method described herein is that the alkylating catalyst shows high activity while displaying high selectivity for the products of interest. In particular, the method of this invention affords improved yields of 2,4,6-trialkylphenols. More specifically, the method of this invention gives a higher yield of 2,4,6-triisopropylphenol relative to prior art methods involving heterogeneous catalysis. Another advantage of this invention is that alkylation with alpha-monosubstituted and alpha, beta-disubstituted olefins occur under conditions where there is little attending oligomerization.